Plants grown in vitro can provide a major source of specialty chemicals, which are plant secondary metabolites. For example, artemisinin, a terpenoid found in the herb, Artemisia annua, is a promising therapeutic for treatment of malaria. However, this highly effective compound is produced by native plants in minute quantities, and by tissue cultured plants at levels significantly less than the best native plants. Efforts to obtain higher production levels in either native plants or their cultured tissues would make therapeutic use of artemisinin on a large scale, a reality.
Taxol is a novel diterpene isolated primarily from the bark of Taxus brevifolia and other species of Taxus. Although knowledge about the sites of taxol biosynthesis and distribution is limited, it is known that the product is next most abundant in the root. Taxol has been shown to be an especially effective antitumor agent. However, there is considerable difficulty in obtaining sufficient quantities of taxol for clinical testing. The yield of taxol from Taxus bark is low (500 gm per 10,000 lbs bark), and native plants are rapidly dwindling in number. Like many woody plants the genus is recalcitrant to propagation by in vitro culture, rooted cuttings, or reseeding for reforestation. Further therapeutic development will depend largely on solving problems of biomass supply. Because Taxus biomass grows so slowly and taxol is produced at low levels in the tissues, the biomass is valuable.
Except for shikonin and berberine, there has been little success in the profitable production of secondary metabolites from plant cultures. The lack of success is due in part to the fact that these chemicals are present only in small amounts within the plant. In addition, in vitro and in vivo cloning of plants, especially woody plants is extremely difficult, because of slow growth rates, reduced or lack of rooting ability, frequent systemic microbial contamination, phenotypic instability, and phenolics build-up. Micropropagation is no different.
There has been some success in the establishment of undifferentiated callus and cell culture lines for the production of secondary metabolites. (See, for example, U.S. Pat. No. 5,019,504 entitled, "Production of taxol or taxol-like compounds in cell culture," by Christen et al.) However, secondary metabolism is frequently linked to differentiation. Therefore, most undifferentiated cultures are not useful for producing secondary metabolites.
Most secondary metabolites of plants accumulate within the plant tissue and are not readily exported into the growth medium. In addition, most secondary metabolites are chemically complex and therefore are difficult to synthesize. Thus, there is a need to develop nondestructive methods whereby products produced by plant tissues can be easily recovered for processing while still retaining the valuable biomass for additional product biosynthesis.